fix($httpBackend): remove Content-Type header on DELETE requests

Sending empty string with `XMLHttpRequest`'s `post` method causes browsers adding unneeded `Content-Type` request header on `DELETE` requests. Chrome adds `Content-Type:application/xml`, Firefox and IE adds `Content-Type text/plain`. Sending `null` insted of empty string fixes the problem.

More detail: https://github.com/angular/angular.js/issues/2149#issuecomment-22664501